It is said that imposing punishment is one of the most difficult things a judge will ever do. Implicit in the imposition of punishment are a number of competing philosophical considerations: retribution, incapacitation, deterrence, and rehabilitation, among others. In many jurisdictions, judges enjoy broad authority over sentencing decisions; in others, judicial discretion is limited by mandatory-minimum penalties or sentencing guidelines. A new, actuarial approach to sentencing (“evidence-based sentencing”) focuses upon the reduction of recidivism, using data to maximize the utilitarian ends of sentencing. Research suggests that statistical assessments outperform clinical judgment of even trained experts. But which variables should judges use when engaging in evidence-based sentencing? Some variables are uncontroversial (e.g., criminal history or role in the offence) but others are legally and/or philosophically problematic (e.g., age, gender, marital status, or race). Aside from the considerable logistical challenges of accurately recording relevant variables, evidence-based sentencing forces courts to confront difficult jurisprudential questions about the goals of punishment, the tension between equity and efficacy, and fundamental concepts of autonomy and free will.